1. Field of the Invention
The present invention relates to an electroacoustic transduction film used for an acoustic device such as a speaker or a microphone, and a manufacturing method thereof.
2. Description of the Related Art
In response to thinning of displays such as liquid crystal displays and organic electroluminescence (EL) displays, speakers used for such thin displays are also required to be lighter and thinner.
In addition, a flexible display using a flexible substrate of plastic or the like has been studied. As a substrate of such a flexible display, for example, a flexible display substrate in which a gas barrier layer and a transparent conductive layer are laminated on a transparent plastic film is disclosed in JP2000-338901A.
The flexible display has superiority in lightweight properties, thinness, flexibility, and the like compared to a display of the related art using a glass substrate, and thus is able to be provided on a curved surface of a cylinder or the like. In addition, the flexible display is able to be contained by being rolled up, and thus does not impair portability even in the form of a large screen. Furthermore, the flexible display has been attracting attention as a display for displaying advertisement, or as a display device such as a personal digital assistant (PDA).
In a case where such a flexible display is used as an image display device-cum-sound generating device which concurrently reproduces an image and a sound as with a television receiver or the like, a speaker which is an acoustic device for generating a sound is necessary.
Here, as the shape of a speaker of the related art, a so-called cone shape such as a funnel-like shape, a spherical dome-like shape, and the like are generally used. However, when such a speaker is embedded in the flexible display described above, there is concern that lightweight properties and flexibility which are the advantages of the flexible display may be impaired. In addition, in a case where the speaker is attached to the outside of the flexible display, it is difficult to carry the flexible display and to dispose the flexible display on a curved wall, and thus there is concern that the aesthetic properties of the flexible display may also be impaired.
Among them, as a speaker which is thin and is able to be integrated with a thin display or a flexible display without impairing the lightweight properties or flexibility, it has been proposed to use a piezoelectric film which has flexibility in the form of a sheet and has a property that stretches and contracts in response to an applied voltage.
For example, in JP2008-294493A, it is described that a piezoelectric film (electroacoustic transduction film) in which electrodes forming a pair are laminated on both surfaces of a piezoelectric film (piezoelectric layer) obtained by performing polarization processing with respect to a monoaxially stretched film of polyvinylidene fluoride (PVDF) at a high voltage is used.
In order to adopt the piezoelectric film as a speaker, it is necessary that a stretching and contracting movement along a film surface is converted into a vibration of the film surface. This conversion from the stretching and contracting movement into the vibration is attained by holding the piezoelectric film in a bent state, and thus the piezoelectric film is able to function as a speaker.
In addition, the applicant proposed an electroacoustic transduction film disclosed in JP2014-14063A as a piezoelectric film which has flexibility in the form of a sheet and is able to stably reproduce a sound with high acoustic quality. The electroacoustic transduction film disclosed in JP2014-14063A includes: a polymer composite piezoelectric body (piezoelectric layer) in which piezoelectric body particles are dispersed in a viscoelastic matrix formed of a polymer material having viscoelasticity at a normal temperature; thin film electrodes formed on both surfaces of the polymer composite piezoelectric body; and protective layers formed on the surfaces of the thin film electrodes.